Synonyms are words which have the same or nearly the same meaning in the same language.
Antonyms are words which have opposite meaning in comparison to the other.
Option D)Resolute is correct as the word vacillating means wavering between different opinions or actions; irresolute. and the word resolute means admirably purposeful, determined, and unwavering.. Both the words are opposite of each other.
The other options are not correct as fascinating, fanaticism , and indolence are not the opposites of the word vacillating.